As a measure against global warming, solar photovoltaic systems are increasingly installed at many places. The total production of photovoltaic devices in the world in 2005 was 1,759 MW, which is an increase by 147% over the previous fiscal year. At present, the most popular photovoltaic device is crystal photovoltaic devices, and photovoltaic devices using single crystal silicon or polycrystalline silicon account for the large part of the production. A photovoltaic device which is called a crystal photovoltaic device uses a silicon wafer as a base, which is formed by forming a large silicon ingot and slicing the silicon ingot thin.
It is estimated that, in a crystal photovoltaic device using single crystal silicon or polycrystalline silicon, a thickness of a silicon wafer which is sufficient to generate photoelectromotive force is about 10 μm. However, a silicon wafer cut out from a silicon ingot has a thickness of about 200 μm to 500 μm. This means that only about 5% of a silicon wafer which is used in a photovoltaic device contributes to photoelectric conversion.
As production of photovoltaic devices increases, shortage in supply of silicon, which is a row material of silicon wafers and steep rise in price of silicon wafers become problems in industry. The production of polycrystalline silicon including those for semiconductors in the world in 2006 was about 37 thousand tons, and polycrystalline silicon which is demanded for solar sells is 11 thousand tons. Production of photovoltaic devices increases every year and the demand has already been tight. In order to increase production capacity of polycrystalline silicon, a large investment is needed and it is difficult to ensure the production which corresponds to the demand. Therefore, it is expected that shortage of silicon wafer supply will continue.
Here, as another type of a photovoltaic device using a single crystal semiconductor substrate, a photovoltaic device using a single crystal semiconductor layer which is thinned is given. For example, Patent Document 1 (Patent Document 1: Japanese Published Patent Application No. H10-335683) discloses a tandem solar cell in which hydrogen ions are implanted into a single crystal silicon substrate, and a single crystal silicon layer which is separated from the single crystal silicon substrate in a layer shape is disposed over a supporting substrate in order to lower the cost and save resources while maintaining high conversion efficiency. In this tandem solar cell, a single crystal semiconductor layer and a substrate are bonded to each other with a conductive paste.
Further, an attempt to directly form the crystal semiconductor layer over a substrate has been conventionally made. For example, a method for manufacturing a silicon thin film photovoltaic device in which crystal silicon film is deposited over a substrate by using a VHF which is higher than 27 MHz and pulse modulated (see Patent Document 2: Japanese Published Patent Application No. 2005-50905). Further, a technique for controlling plasma treatment conditions to optimize a concentration of a dopant in crystal grains and crystal grain boundaries when a thin film polycrystalline silicon film is formed by a plasma CVD method over a special electrode called a texture electrode which has minute unevenness on its surface (see Patent Document 3: Japanese Published Patent Application No. 2004-14958).